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treated in this program as attributes, parameterised by absolute time,
but in relational algebra would be more properly treated as a relationship
to "observance" entities.  It is intended that a future version of this
program will be able to search among observances within a canonical
timezone, treating observances as first-class entities.

A canonical timezone is related to one or more timezone, which is
what defines where the clock behaviour is used.  See the notes in the
timezone entry above about the namespace confusion.  Via the timezones,
a canonical timezone is related to zero or more areas, zero or more
regions, and zero or more countries.

=back

In relational algebra terms, the listing facility selects from a join
between all of these entity types.  With only distinct result rows being
shown, some of the entity types are typically irrelevant, and can be seen
as being implicitly dropped from the join.  The join is fully outer,
meaning that all of the underlying entities are available for output
even where they don't relate to any other entities through the join.
(This is part of what the exceptions are about.)

=head2 Examples

=over

=item *

List countries in a particular area, as an intermediate step in a user
selecting a timezone:

    olson list a=pacific c cn +cn

=item *

List timezones in a particular country, as the final step in a user
selecting a timezone:

    olson list c=ki z rd +rd

=item *

Which offsets from UT could be meant by "CST"?

    olson list i@now=CST o@now

This misses out on any timezones where "CST" is used in part of the year
but not right now.

=item *

Which timezones have jumped westward across the International Date Line
since 1900?

    olson list 'o@1900z < -08' 'o@now > +08' k

=back

=head1 SEE ALSO

L<DateTime::TimeZone::Olson>,
L<Time::OlsonTZ::Data>

=head1 AUTHOR

Andrew Main (Zefram) <zefram@fysh.org>

=head1 COPYRIGHT

Copyright (C) 2012 Andrew Main (Zefram) <zefram@fysh.org>

=head1 LICENSE

This module is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.

=cut

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